(树直径) bzoj 1509

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
vector<int> e[200005],w[200005];
int n,m;
long long dista[200005],distb[200005];
bool vis[200005];
int bfs(int u,long long dist[])
{
    long long ret=0;
    int pos=0;
    memset(vis,0,sizeof(vis));
    dist[u]=0;
    queue<int> q;
    q.push(u);
    vis[u]=1;
    while(!q.empty())
    {
        int x=q.front();
        q.pop();
        for(int i=0;i<e[x].size();i++)
        {
            int v=e[x][i];
            if(vis[v]) continue;
            dist[v]=dist[x]+w[x][i];
            vis[v]=1;
            q.push(v);
            if(dist[v]>ret)
            {
                pos=v;
                ret=dist[v];
            }
        }
    }
    return pos;
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        int x,y,z;
        scanf("%d%d%d",&x,&y,&z);
        e[x].push_back(y);
        e[y].push_back(x);
        w[x].push_back(z);
        w[y].push_back(z);
    }
    int a=bfs(1,dista);
    int b=bfs(a,dista);
    bfs(b,distb);
    long long ans=0;
    for(int i=1;i<=n;i++)
        ans=max(ans,min(dista[i],distb[i]));
    ans+=distb[a];
    printf("%lld\n",ans);
    return 0;
}